In the model described in Figure 1, the user-protocol interpreter
initiates the control connection. The control connection follows
the Telnet protocol. At the initiation of the user, standard FTP
commands are generated by the user-PI and transmitted to the
server process via the control connection. (The user may
establish a direct control connection to the server-FTP, from a
TAC terminal for example, and generate standard FTP commands
independently, bypassing the user-FTP process.) Standard replies
are sent from the server-PI to the user-PI over the control
connection in response to the commands.

The FTP commands specify the parameters for the data connection
(data port, transfer mode, representation type, and structure) and
the nature of file system operation (store, retrieve, append,
delete, etc.). The user-DTP or its designate should "listen" on
the specified data port, and the server initiate the data
connection and data transfer in accordance with the specified
parameters. It should be noted that the data port need not be in
the same host that initiates the FTP commands via the control
connection, but the user or the user-FTP process must ensure a
"listen" on the specified data port. It ought to also be noted
that the data connection may be used for simultaneous sending and
receiving.

In another situation a user might wish to transfer files between
two hosts, neither of which is a local host. The user sets up
control connections to the two servers and then arranges for a
data connection between them. In this manner, control information
is passed to the user-PI but data is transferred between the
server data transfer processes. Following is a model of this
server-server interaction.

The protocol requires that the control connections be open while
data transfer is in progress. It is the responsibility of the
user to request the closing of the control connections when
finished using the FTP service, while it is the server who takes
the action. The server may abort data transfer if the control
connections are closed without command.

The Relationship between FTP and Telnet:

The FTP uses the Telnet protocol on the control connection.
This can be achieved in two ways: first, the user-PI or the
server-PI may implement the rules of the Telnet Protocol
directly in their own procedures; or, second, the user-PI or
the server-PI may make use of the existing Telnet module in the
system.

Ease of implementaion, sharing code, and modular programming
argue for the second approach. Efficiency and independence
argue for the first approach. In practice, FTP relies on very
little of the Telnet Protocol, so the first approach does not
necessarily involve a large amount of code.